1. Field of the Invention
The present invention relates to an auto light-control system. More particularly, the present invention relates to an auto light-control system which is suitable for the control of a brightness of a lighting device, such as a head light, in accordance with an ambient brightness.
The present application is based on Japanese Patent Applications Nos. Hei. 11-49524 and 2000-17001, which are incorporated herein by reference.
2. Description of the Related Art
An auto light-control system shown in FIG. 5 has been known. The auto light-control system includes a light sensor 1, a CPU (microprocessor) 2, relays 3 and 4, FET 5, a clear lamp (tail lamp) 6, a head lamp (head light) 7, and a head lamp 8.
The light sensor 1, as shown in FIG. 6, is designed such that it senses an ambient brightness and produces an analog signal having a signal level which corresponds to the ambient brightness. The CPU 2 converts an analog signal received from the light sensor 1 into a digital signal, and compares a detection level of the light sensor 1 with a preset level. When as the result of the comparison, the detection level exceeds a preset level, the CPU 2 drives the relay 3 to turn on the clear lamp 6. When the detection level of the light sensor 1 exceeds another preset level b2, the CPU 2 drive the relay 4 to turn on the head lamp 7. At the same time, the CPU 2 applies a PWM signal to the FET 5 to turn on the FET 5, whereby the head lamp 8 is continuously turned on.
Thus, the auto light-control system uses the relay-basis control for the turning-on of the clear lamp 6 and the head lamp 7 and the semiconductor-basis control for the turning-on of the head lamp 8.
In the related auto light-control system, the control system including the CPU 2, for example, is indispensably used for the brightness control of the clear lamp 6 and the head lamps 7 and 8. For this reason, it is difficult to simplify the circuit arrangement.
An object of the present invention is to provide an auto light-control system which can control a brightness of the lamp as a load in accordance with an ambient brightness by use of a simple circuit arrangement.
To achieve the above object, according to the first aspect of the present invention, there is provided an auto light-control system which comprises a light sensor which senses an ambient brightness, and produces an analog signal representative of the ambient brightness, an analog-to-digital converting device which converts the analog signal output from the light sensor into a digital signal with a pulse width corresponding to a signal level of the analog signal, and outputs the digital signal as a drive signal, and a first switching device which is inserted in a power supply circuit connecting a load and a power supply, and which is adapted to conduct in response to the drive signal to close the power source circuit. In the auto light-control system, an output signal of the light sensor is applied through the analog-to-digital converting device to the first switching device, whereby the first switching device is always on/off controlled. Therefore, a brightness of the light as the load is controlled in accordance with an ambient brightness. Accordingly, a brightness of the load, e.g., the lighting device, can be controlled in accordance with an ambient brightness using a simple circuit arrangement, not using the CPU.
According to the second aspect of the present invention, it is preferable that the auto light-control system further comprises a reference resistor which receives a current from the power supply to generate a reference voltage, and a second switching device which is inserted in a shunt circuit connecting the power supply and the reference resistor, and which is adapted to conduct in response to the drive signal to close the shunt circuit. In accordance with this configuration, the drive of the first switching device can be controlled in a stable state.
According to the third aspect of the present invention, it is preferable that the auto light-control system further comprises a drive stopping device which stops the analog-to-digital converting device from outputting the drive signal when an output voltage of the first switching device is compared with the reference voltage and a difference between the output voltage and the reference voltage is out of a tolerance. Accordingly, since the drive stopping device stops the drive of the first switching device in an emergency, protection of the switching device is secured in an abnormal state, e.g., a load short-circuiting.